Refractory compositions containing carbon

ABSTRACT

Refractory compositions based on aluminum nitride, boron nitride, and at least one refractory boride, nitride, carbide or silicide. A small amount of carbon, such as about 0.5 to 3 percent by weight, is added to the composition before hotpressing. The resultant hot-pressed bodies have improved properties especially in the length of their life when used as aluminum-vaporizing crucibles.

States atent James [4 Mar. 14, 1972 [54] REFRACTORY COMPOSITIONS [56] References Cited CONTAINING CARBON UNITED STATES PATENTS [72] Derek James England 2,814,566 11/1957 Glaser 106/43 [73] Assignee; United States Borax 8; Chemical Corpora- 2,839,413 6/1958 Taylor ..l06/65 tion, Los Angeles, Calif. 3,251,700 5/1966 Mandorf.... 106/65 3,256,103 6/1966 Roche et a1. .....lO6/55 [22] 1969 3,408,312 /1968 Richards 106/55 [21] Appl. N0.: 888,942 3,514,271 5/1970 Yates 106/55 Primary Examiner-James E. Poer [30] Fore1gn Apphcation Pnonty Data Attorney james R Thornton Jan. 9, 1969 Great Britain ..1,439/69 [57] ABSTRACT [52] U.S.Cl ..l06/56, 106/43, 106/57,

106/65 106/66 252/504, 252/506 252/507, Refractory composmons based on alummurn mtnde, laoron 252/508 nitnde, and at least one refractory bonde, mtnde, carb1de or [511 1m. (:1. ..C04b /52 c0415 35/58 silicide A cam", such as to 3 [58] Field 61 Search 106/43, 55, 5'6, 57, 65 66; by weigh" is added befm 252/504, 505,508 pressing. The resultant hot-pressed bodies have improved properties especially in the length of their life when used as aluminum-vaporizing crucibles.

9 Claims, No Drawings This invention relates to refractory compositions having particular utility for making crucibles of the type used for vaporizing aluminum, and the invention provides such compositions and provides also shaped articles such as aluminumvaporizing crucibles made from the composition and a method of vaporizing aluminum from a crucible made of the compositions of the invention,

Aluminum-vaporizing crucibles, commonly called boats, are used in the process of depositing a thin film of aluminum on a work surface, in which process a rod or wire of aluminum is passed continuously onto the vaporizing surface of the boat which is resistance-heated by passage of an electric current therethrough so as to vaporize the aluminum under partial vacuum, the aluminum vapor then being condensed onto the work sudace to form the desired film.

According to the present invention there is provided an improvement in refractory compositions comprising approximately equal parts of two components e.g., from 40 to 60 percent of either component) of which the first component is a mixture of aluminum nitride and boron nitride (the aluminum nitride being the major part of the first component and preferably comprising 6080 percent thereof) while the second component is at least one boride, nitride, carbide or silicide of a metal such that the compound is within the class of substances known as refractory hard metals. The improvement comprises the incorporation, as a third component of from 0.5 to about 3% by weight of the total composition of carbon or of a substance which can be heated to provide carbon.

By the term refractory hard metals we mean in the first place the borides, carbides, nitrides, and silicides of titanium or zirconium and also the other metals such as tungsten, hafnium, molybdenum, tantalum, vanadium, niobium, and chromium which are commonly called refractory metals. However, chromium nitride is unsuitable and is, therefore, excluded.

The second component is preferably titanium diboride or zirconium diboride or a mixture thereof. The third component can be included in the mixture in the form of graphite or lampblack. However, finely divided graphite is preferred. Altematively, the third component may be present by inclusion of carbonaceous resin, such as furane, such that when the mixture is hot pressed or sintered the resin is converted to carbon.

The invention provides also a method of making refractory articles which comprise subjecting a composition of the inven tion to pressure and to heat, preferably simultaneously as in hot pressing. One advantageous method of operating according to the invention is to cold-press the composition to desired shape, coat the shapes with boron nitride to prevent their sticking together and then hot-press a number of the shapes togetherv As a result of the hot pressing to form the sintered body, the carbon appears to react with the other components, such as boron nitride to form some boron carbide.

A typical composition according to the invention has the following components (by weight):

aluminum nitride 33% boron nitride titanium diboride 50% graphite 2% This composition is made by adding to each other the requisite amounts of the four ingredients in fine powder form and intimately dispersing the powders to form a fine grey powder. The powder composition can be fabricated by hot pressing into blocks. Two samples of this composition were hot pressed to respectively a low density (about 90 percent of the theoretical) and a high density (more than 95 percent theoretical).

Suitable conditions for hot pressing the compositions of the invention are at a temperature of about 1,800 C. and a pressure of about 2,700 p.s.i.

These blocks were compared with control blocks made in identical fashion from a composition containing the same proportions of aluminum nitride, boron nitride and titanium diboride but containing no carbon. The blocks were compared for their length of life (in hours) when used as an aluminumvaporizing crucible, for their machinability, for the ease with which the composition can be pressed, and for their electrical properties. Results were as follows:

High Density Low density Life with carbon 33 33 Life without carbon 29 I7 Electrical properties with carbon good poor properties without carbon poor poor Machinability with carbon more easy difficult Machinability without carbon dilTlCUll easy Pressing with carbon fair good Pressing without carbon not so fair good it is apparent that the advantage of the presence of carbon is the long life given to the blocks, especially when the blocks are of low density which facilitates machining.

A boat or crucible of this invention may consist of an elongated bar of flat cross section having a cavity machined out along one face thereof and in use is clamped at respective ends between two electrical contacts and electric current is passed between the contacts through the boat while aluminum is fed into the cavity and vaporized therefrom, the operation being effected under vacuum of 1 M0 to 5 X 10*torr.

Various changes and modifications of the invention can be made and, to the extent that such variations incorporate the spirit of this invention, they are intended to be included within the scope of the appended claims.

What is claimed is:

l. A refractory composition consisting essentially of about equal parts by weight of a. a mixture of about 60-80 percent of aluminum nitride and about 2040 percent of boron nitride, and

b. at least one refractory hard metal selected from the refractory borides, nitrides, carbides and silicides, and about 0.5 to 3 percent by weight of carbon.

2. A refractory composition according to claim 1 in which b) is selected from the group consisting of titanium diboride, zirconium diboride, and mixtures thereof.

3. A refractory composition according to claim 1 which has the composition, by weight,

33% aluminum nitride 15% boron nitride 50% titanium diboride 2% graphite 4. A refractory composition according to claim 1 in which 12) is a refractory boride.

5. A refractory composition according to claim 1 in which said carbon is graphite.

6. In the method for forming a sintered refractory body by forming an admixture of about equal parts by weight of a. a mixture of about 60-80 percent of aluminum nitride and about 20-40 percent of boron nitride, and

b. at least one refractory hard metal selected from the refractory borides, nitrides, carbides and silicides, and hot-pressing said admixture to provide a sintered body, the improvement which consists of incorporating in said admixture about 0.5 to 3 percent by weight of a member of the group consisting of carbon and resins convertible to said amount of carbon upon heating.

7. The method according to claim 6 in which b) is selected from the group consisting of titanium diboride, zirconium diboride, and mixtures thereof.

8. The method according to claim 6 in which said carbon is graphite.

9. A sintered refractory composition in the shape of a crucible adapted for vaporization of aluminum consisting essentially of 33% aluminum nitride boron nitride 50% titanium diboride 2% carbon,

said percentages by weight. 

2. A refractory composition according to claim 1 in which b) is selected from the group consisting of titanium diboride, zirconium diboride, and mixtures thereof.
 3. A refractory composition according to claim 1 which has the composition, by weight, 33% aluminum nitride 15% boron nitride 50% titanium diboride 2% graphite
 4. A refractory composition according to claim 1 in which b) is a refractory boride.
 5. A refractory composition according to claim 1 in which said carbon is graphite.
 6. In the method for forming a sintered refractory body by forming an admixture of about equal parts by weight of a. a mixture of about 60- 80 percent of aluminum nitride and about 20- 40 percent of boron nitride, and b. at least one refractory hard metal selected from the refractory borides, nitrides, carbides and silicides, and hot-pressing said admixture to provide a sintered body, the improvement which consists of incorporating in said admixture about 0.5 to 3 percent by weight of a member of the group consisting of carbon and resins convertible to said amount of carbon upon heating.
 7. The method according to claim 6 in which b) is selected from the group consisting of titanium diboride, zirconium diboride, and mixtures thereof.
 8. The method according to claim 6 in which said carbon is graphite.
 9. A sintered refractory composition in the shape of a crucible adapted for vaporization of aluminum consisting essentially of 33% aluminum nitride 15% boron nitride 50% titanium diboride 2% carbon, said percentages by weight. 